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She keeps a diary in English.
Literal
She [topic-は] English [in-で] diary [object-を] is-attaching.
日記をつける ('to keep a diary') is the standard collocation — the verb is 付ける ('to attach / put'), here in the figurative sense of 'putting down / committing to record.' The same つける anchors a small constellation of recording verbs: 印をつける ('mark'), 名前をつける ('give a name'). で marks the medium of expression — 'in English,' just as 日本語で書く ('write in Japanese') uses で for the language. ~ている here is the habitual reading: she keeps a diary as an ongoing practice, not 'is currently keeping' in this exact moment.